1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to improving image display of a LCD in which a source line is formed overlapping pixel electrodes.
2. Description of the Related Art
A vertical orientation type LCD comprising liquid crystal having negative anisotropy of dielectric constant and a vertical orientation film has been proposed in, for example, JPA H06-301036. An LCD of this type is described below.
FIG. 1A is a plan view showing such a LCD, and FIG. 1B shows a cross-sectional view taken along line A-A′ of FIG. 1A. A gate line 51 is formed on a first substrate 50, and a gate insulating film 52 is formed covering the gate line 51. The gate line 51 comprises gate electrodes 51a within a portion of each pixel. Over these portions, poly-silicon film is provided in the form of discrete islands so as to cross over the gate electrodes 51a. The poly-silicon film is then doped with impurities to create, together with the gate electrodes 51a, thin film transistors (TFT) 54. An interlayer insulating film 55 is formed over these components, and a data line 56 is superimposed on the interlayer insulating film 55. Subsequently provided is a planarizing film 57, and pixel electrodes 58 composed of ITO (indium tin oxide) are formed thereon. Each pixel electrode 58 is connected to a TFT 54 via a contact hole opened through the interlayer insulating film 55 and the planarizing film 57. The data line 56 is formed overlapping under the pixel electrode 58. The data line 56 is connected to the source regions of the TFTs 54 and supplies electric charges to the pixel electrodes 58 when the gate electrodes 51a are turned on. Formed over the pixel electrodes 58 is a vertical orientation film 59 made of an organic material such as polyimide or of an inorganic silane material. Rubbing processing is not performed on the vertical orientation film 59.
Provided on a second substrate 60 arranged opposing the first substrate 50 are color filters 66 in positions corresponding to the pixel electrodes 58. Each color filter is colored either one of red (R), green (G), and blue (B), or alternatively, cyan, magenta, and yellow. Over the color filters 66, a common electrode 61 composed of ITO or a similar material is formed extending in a region over a plurality of pixel electrodes 58. A vertical orientation film 62 identical to the one disposed on the first substrate 50 is provided over the common electrode 61. Orientation control windows 63, i.e. regions where no electrode is present, are formed in the common electrode 61. The orientation control windows 63 may have a shape of two letter Y's connected at their bottoms.
Liquid crystal 70 is sealed between the first and second substrates 50,60. The orientation of liquid crystal molecules is controlled in accordance with the strength of electric field generated by a voltage applied between the pixel electrodes 58 and the common electrode 61. On the outer side of the first substrate 50 and the second substrate 60, polarizers (not shown) are arranged such that their polarization axes are perpendicular to one another. Linearly polarized light that travel between the polarizers is modulated while passing through the liquid crystal 70 controlled to different orientations in the respective display pixels. The light is thereby controlled to achieve desired transmittance.
The liquid crystal 70 has negative anisotropy of dielectric constant. That is, the liquid crystal 70 has the property to orient such that the longitudinal axes of its molecules become perpendicular to the direction of the electric field. The vertical orientation films 59,62 control the initial orientation of the liquid crystal 70 to the vertical direction. When no voltage is applied, the liquid crystal molecules are oriented vertically with respect to the plane of the vertical orientation films 59,62. In this case, the linearly polarized light that has passed through one of the polarizers passes through the liquid crystal layer 70, but is obstructed by the other polarizer. The resulting display is seen as black.
In the above-described arrangement, a voltage is applied between a pixel electrode 58 and the common electrode 61 to generate electric fields 64,65 which tilt the liquid crystal molecules. At end portions of the pixel electrode 58, electric field 64 curves from the pixel electrode 58 towards the common electrode 61. Similarly due to the absence of any electrodes, electric field 65 curves towards the pixel electrode 58 at edges of an orientation control window 63. The curved electric fields control the orientation of the liquid crystal by tilting the molecules towards the inboard of the pixel electrode 58 and towards the orientation control window 63.
In regions directly underneath orientation control windows 63, no electric field is generated during voltage application because the common electrode 61 is absent. Liquid crystal molecules are therefore fixed in the initial orientation state, namely, the vertical direction. This allows regions of the liquid crystal on the respective sides of the orientation control window 63 to be oriented in opposing directions due to the continuous property of liquid crystal. As a result, a display with a broad viewing angle can be obtained.
The controller of the liquid crystal orientation is not limited to orientation control windows 63. Alternatively, slope portions may be disposed in the vertical orientation films 59, 62 on the sides contacting the liquid crystal 70. Details concerning this point are found in Japanese Patent Application No. Hei 6-104044 (JPA H07-311383) filed by the present applicant.
The voltage application scheme of the LCD is next explained. FIG. 2 is a timing chart showing voltages applied to gate lines 51 and data lines 56, and voltages of pixel electrodes driven by those applied voltages. FIGS. 2(a), 2(b), and 2(c) illustrate the voltages applied to mth gate line, m+1th gate line, and a data line, respectively. FIG. 2(d) indicates the voltage of a pixel electrode controlled by the mth gate line and the data line. FIG. 2(e) indicates the voltage of a pixel electrode controlled by the m+1th gate line and the data line. During one horizontal synchronization period (referred to hereinafter as 1H), a voltage is applied to the mth gate line to switch it on. When the mth gate line is switched on, TFTs of pixel electrodes in the associated row are accordingly turned on. During 1H, voltages according to an image to be displayed are applied to the respective data lines, and each of these voltages is retained by a pixel electrode in that row. In the next 1H, the mth gate electrodes are turned off while the m+1th gate electrodes are turned on. Accordingly, TFTs of pixel electrodes associated with the m+1th gate electrodes are turned on. Voltages in the data lines 56 are then retained by the pixel electrodes in this row. Similar procedures are repeated to apply voltages to each row of pixel electrodes 58 and to drive associated liquid crystal, thereby displaying an image. During these procedures, the direction of electric field is inverted for each adjacent rows to prevent degradation of liquid crystal. Specifically, the pixel electrodes in the row controlled by mth gate line may be applied with voltage Vhigh (10V) higher than the potential Vc of the common electrode 63 (6V, for example) by a predetermined potential (4V, for example), while applying an inverted voltage Vlow (2V), i.e. a voltage lower than the potential Vc of the common electrode 63 by the predetermined potential, to the pixel electrodes of the adjacent row. When again applying a voltage to the pixel electrodes of the row associated with mth gate line, the inverted voltage of the previously applied voltage, namely, Vlow, is applied. Such a voltage application scheme is referred to as the line inversion scheme. As the voltages applied to the pixel electrodes are alternately inverted for each row using the potential Vc of the common electrodes 63 as the point of inversion, the electric fields generated according to the line inversion scheme have uniform shapes with inverted directions for each row.
As mentioned above, data lines 56 overlap pixel electrodes 58 in a vertical orientation type LCD, generating parasitic capacitance CSD between the data lines 56 and the pixel electrodes 58. Further, when employing the line inversion scheme, each data line 56 is applied with voltages similar to an alternating current as shown in FIG. 2(c). Such voltages of the data line 56 affect the pixel lectrodes 58 as noise. As a result, voltages retained by the pixel electrodes 58 cannot be maintained at the values of Vhigh or Vlow applied, and receive influences from the voltages applied to the data line 56 as shown in FIGS. 2(f) and 2(g). The voltages retained by the pixel electrodes 58 would therefore be at values substantially lower than Vhigh or higher than Vlow, and accordingly, a significant reduction in potential difference between the pixel electrodes 58 and the common electrode 61 cannot be avoided.
When the potential difference between the pixel electrodes 58 and the common electrode 61 is reduced, an electric field of sufficient strength cannot be applied to the liquid crystal 70. The contrast of the LCD would then degrade due to insufficient driving of the liquid crystal.
The present applicant proposed in Japanese Patent Application No. Hei 10-337840 a technique of arranging data signal lines 56 for supplying display signals to the pixel electrodes 58 via thin film transistors in positions overlapping the orientation controllers 63 shown in FIG. 1. This technique does not constitute prior art for the present invention. Adoption of this arrangement can prevent light leakage or other negative influences due to disturbances in liquid crystal orientation in regions corresponding to orientation control windows where voltage cannot be controlled, without substantially reducing the aperture ratio. More specifically, light transmitting through a data line 56 attenuates by a fixed ratio. In addition, when in the “normally black mode”, the liquid crystal beneath an orientation control window 63 does not allow light to pass through even during voltage application because its initial orientation is maintained. When a data line 56 overlaps a pixel electrode 58 as shown in FIG. 1, for example, the aperture ratio reduces in proportion to the amount of the overlap. However, by arranging the data line 56 to overlap the orientation control window 63 according to the proposed technique, regions originally having strong light-shielding property would be overlaid on one another. In this way, although the light-shielding property in such an overlapped region becomes stronger, the substantial aperture ratio of the pixel can be effectively minimized.
However, when a data line 56 is formed overlapping the orientation control window 63, the wiring length of the data line 56 within a pixel is increased compared to a case when the data line 56 is linearly formed as shown in FIG. 1. Parasitic capacitance between the data line 56 and the pixel electrode 58 therefore becomes increased, and reduction of the potential difference due to the above-described noise may become further notable.
This problem may similarly exist when a data line is formed overlapping pixel electrodes in LCDs other than ones having orientation control windows, such as a LCD with slope portions disposed on an orientation film contacting the liquid crystal for orientation control described in the above-mentioned JPA H07-311383.